1. Field of the Invention
The invention relates generally to charge-coupled devices and in particular to a device for injecting a highly controlled level of charge into a charge-coupled device (CCD) register.
2. Description of the Prior Art
Charge-coupled devices (CCDs) comprise a relatively new technology for storage and transfer of information. They operate so as to store packets of charge in sites or cells within the CCDs. The transfer of charge from one cell to another may be done only in a serial manner. CCDs may allow for significant cost reductions in mass storage systems, if high densities are realized. However, their application is available only when serial storage systems are utilizable.
Charge-coupled devices comprises near conservative systems (some charge may be lost to substrate or ground by leakage), that is, charge is not and cannot be generated within the device. Charge is injected into a CCD at its input terminal and thereafter may be stored or transferred within or through the device. During storage and transferring stages, charge is dispersed.
To achieve desired economies, CCDs must be of very high density. This requires, in part, that each site within the CCD is relatively small. Small cites can hold only small amounts of charge. However, (differential) charge amplifiers used to sense charge emitted from CCD storage registers require good 1 and 0 definition to properly operate. Accordingly, it is of prime importance to have a system for injecting charge into a CCD which is capable of controlling the levels of injected charge in a highly precise manner, that is, the tolerance for levels of injected charge is extremely narrow.
The charge injector must act as a buffer between system level voltages and CCD charge levels. In effect, it is a converter from these relatively high voltage levels corresponding to logic levels at its input and the CCD levels at its output. Input levels may be in the order of 10 or 12 volts whereas output levels are on the order of approximately 50 to 230 Femto Coulombs. Furthermore, the system level voltages may have wide variation. Therefore, the device must be relatively insensitive at its input while still having higly controlled output charge levels.
In order to inject levels of charge into a CCD, it is theoretically possible to attach a current source as the input to the CCD, and gate this current source so as to control the amount of charge sent to the CCD. In this manner, current would flow into the device, filling up (or injecting) charge to a predetermined level. Unfortunately, this approach is strictly theoretical in nature and is not applicable for the required function. This is because a current source of an extremely precise magnitude would be required and would have to be gated in such a highly-controlled fashion. Such degrees of precision exceed realistic tolerance levels of known current sources.
Another method of injecting charge into a CCD is by attaching a fixed voltage to the device, and by regulating an electrode acting as a gate coupling the voltage to the device. An example of this method is given in a patent entitled, "Input Circuits for Charge Coupled Circuits", invented by Kosonocky, U.S. Pat. No. 3,760,202, issued Sept. 18, 1973. However, methods such as those taught by Kosonocky are only applicable where relatively large cells are utilized and can tolerate larger amounts of uncertainty in the injected signals than would be applicable for the present invention. The method utilized by Kosonocky is generally referred to as the "fixed voltage technique" and has been utilized by others in the prior art. In the fixed voltage method, a fixed reference voltage is connected to the first cell of the CCD via an input control gate. By applying appropriate clock pulses to the input control gate, charge is transferred into the first cell site. However, the amount of charge which is transferred into the cell site is dependent on the level of the reference voltage. Unfortunately, the reference voltage cannot be regulated with the precision required for this application. Furthermore, the input control gate also stores charge in the region of its electrode. The amount of charge underneath this gate may vary and its flow may be unregulated and uncertain. Depending upon the size of the input control gate with respect to cell size, this may present a problem in its utilization. In a system requiring very high density, as required for CCD applications, the control gate must be small, generally, approximately the size of a typical cell site. A control gate of this magnitude may not be gated with the desired precision. Furthermore, it holds charge of levels which may effect the amount of charge transferred into the first cell site. Accordingly, other methods of charge injection are required for the present invention and purposes.
It has been suggested to utilize a clocked source method of charge injection for CCDs. This method provides for a variable (clocked) voltage to be connected to the first cells of the register via an input control gate. This concept was suggested in an article entitled, "Staggered Oxide C4D Structure With Clocked Source Repeater", by W. E. Tchon and J. S. T. Huang, December, 1973, IEDM, Washington, D.C.